1. Field of the Invention
The present invention relates to an electrostatic sensor and an occupant detecting device having the electrostatic sensor.
2. Description of Related Art
JP-A-H11-271463 discloses an occupant detecting device having a mat-shaped electrostatic sensor and a detection electronic control unit (ECU). The sensor has a main electrode arranged in a vehicle seat, and a weak electric field is generated between the main electrode and a vehicle chassis. A disturbance of the electric field is detected, and the detected disturbance is output as a current or voltage.
For example, when the vehicle seat is vacant, air is located between the main electrode and the vehicle chassis. When a child restraint system (CRS) is mounted on the vehicle seat, the CRS is located between the main electrode and the vehicle chassis. When a human adult is seated on the vehicle seat, a body of the adult is located between the main electrode and the vehicle chassis.
Air has a specific dielectric constant of about 1. The CRS has a specific dielectric constant of about 2-5. The body of the adult has a specific dielectric constant of about 50. That is, the specific dielectric constant is different among the air, CRS, and human body. Therefore, a capacitance between the main electrode and the vehicle chassis is varied when a kind of object located between the main electrode and the vehicle chassis is changed.
The disturbance of the electric field is generated by a difference of the capacitance, and is output as a current or voltage. The detection ECU performs an occupant determination based on the output current or voltage. Specifically, the detection ECU determines the vehicle seat to be vacant or to have the CRS or adult. An airbag ECU allows or prohibits an inflation of airbag based on the determination of the detection ECU. When the vehicle seat is vacant, or when a CRS is mounted on the vehicle seat, the inflation of airbag is prohibited. When an adult is seated on the vehicle seat, the inflation of airbag is allowed.
JP-A-2006-27591 discloses an electrostatic sensor to detect liquid such as water contained in a vehicle seat. When the vehicle seat contains water, an occupant determination becomes difficult, because water has a specific dielectric constant of about 80, which is larger than that of a human body. Therefore, the electrostatic sensor has a sub electrode to detect water, other than a main electrode. A disturbance of an electric field between the sub electrode and the main electrode is output as a current or voltage. Thus, water contained in the vehicle seat can be detected, such that a vehicle seat having an occupant can be more clearly distinguished from the wet vacant seat.
JP-A-2006-201129 discloses an electrostatic sensor having a pair of electrodes. A current flowing between the pair of electrodes is reduced when a vehicle seat is vacant. Due to the current reduction, an occupant seated on the vehicle seat can be accurately detected. Specifically, the electrostatic sensor has a capacitance-reducing guard electrode, other than a main electrode. If a sine wave corresponding to a high frequency low voltage is applied between the guard electrode and a vehicle ground (GND) from a detection ECU, capacitors are defined among the main electrode, human body and vehicle GND. The detection ECU detects a current corresponding to a capacitance of the capacitor.
JP-A-2003-520344 discloses an occupant sensor, and an electrode area of the sensor is made larger. In this case, an occupant seated on a vehicle seat can be more easily and accurately detected.
The occupant sensor has a flat shape, and is arranged inside of the vehicle seat at an approximately center position. However, the occupant sensor may have resistance distribution asymmetrical in a left-and-right direction of the vehicle seat relative to an imaginary center line linearly extending in a front-and-rear direction of the vehicle seat. Therefore, detection error may be generated by a seating position deviation of the occupant, and accuracy for determining the occupant may be lowered.